The Human Immunodeficiency Virus Type l (HIV-l) is a complex retrovirus that expresses at least 9 different genes in a temporally-regulated manner. The viral protein Rev (regulator of expression of the virion) regulates the expression of viral late genes, necessary for virion production. Rev is absolutely required for HIV- l replication: Proviruses that lack Rev function remain transcriptionally active, but fail to generate new viral particles. Studies of other complex retroviruses have revealed that Rev-like regulatory proteins are a common feature of lentiviruses as well as HTLV I. The studies proposed here will focus on characterizing the functional interactions required for the in vivo function of HIV- l Rev and related viral regulatory proteins. Using a phylogenetic approach to identify and characterize the common features of this family of post-transcriptional regulatory proteins, it will be possible to define the minimal essential functions required by all Rev- like transactivators. The most common feature of this class of viral regulatory proteins is a functional effector domain which interacts with cellular cofactor(s) facilitating function. This proposal aims to determine the essential requirements of functional Rev-like effector domains by identifying and characterizing these domains from the family of identified Rev-like viral regulatory proteins. Functional effector domains can be identified by their ability to restore the function of an effector domain mutant of HIV-l Rev by direct fusion. The interactions between the effector domain and endogenous cellular cofactor(s) will be characterized in vivo using a nuclear and effector domain specific cross-species competition assay. Effector domains in endogenous proteins will be identified using an effector domain complementation system, employing a cell line generated to allow selection of a complemented effector domain mutant of HIV- l Rev. A third functional region recently identified in both HIV-1 Rev and HTLV-I Rex by complementation studies will be characterized which will also determine if this activity is a common feature of the members of the family of Rev-like transactivators. Understanding these interactions should facilitate the characterization of cellular factors involved in the function of Rev-like transactivators and could allow the identification of endogenous Rev-like proteins. This basic knowledge should lay the groundwork for novel therapeutic approaches targeting Rev function and aimed at delaying the onset of AIDS in infected individuals. From a clinical perspective, Rev is an attractive target for directed therapy because it is absolutely required for viral replication.